1. Field of the Invention
The present invention relates generally to an apparatus for actively mode-locking a laser using a fiber segment containing Fiber Bragg gratings.
2. Description of the Prior Art
Lasers, which provide a stream of light at a particular frequency (or wavelength, in the case of light), are transformed into energy pulses by a process referred to as mode-locking. These energy pulses can then be directed into fiber cable for use in various applications, such as in telecommunications.
However, fiber lasers, i.e., those lasers used in conjunction with fiber cable, that are passively mode-locked are difficult to precisely synchronize at a particular frequency, such as, for instance, clock frequencies used for signals in the OTDM and WDM channels. Actively mode-locked fiber lasers, on the other hand, offer increased stability which, in turn, provides for a more precise synchronization, i.e., synchronizing optical pulses to external electrical sources. As such, actively mode-locked fiber lasers support greater applications.
The conventional method for actively mode-locking a fiber laser requires an external device. such as an electro-optic modulator (xe2x80x9cEOMxe2x80x9d), e.g., a lithium niobate waveguide amplitude modulator, physically located between the laser and the fiber, to modulate the amplitude of the incoming laser source and then pass the modulated source into the fiber.
Unfortunately, a result of using external EOMs is that insertion losses occur. In addition, the cost of EOMs becomes more important with the prolific increase in fiber applications, especially those applications that require all-fiber active devices, such as those required by hybrid WDM/OTDM systems. It is therefore highly desirable for a less expensive modulated fiber-laser and for a modulated fiber-laser with decreased insertion loss.
It is therefore an object of the present invention to provide an improved method for actively mode-locking a fiber laser without utilizing an external EOM.
It is a further object of the present invention to use a fiber cable segment containing a doped gain medium, a first FBG, and a second FBG, wherein a resonant cavity is created so that a fiber laser can be actively mode-locked.
It is yet another object of the present invention wherein the first FBG is a narrowband FBG and the second FBG is an electro-optically tunable FBG.
It is a further object of the present invention wherein the center frequency of the tunable FBG can be adjusted to produce a single mode-locked output frequency.
It is yet another object of the present invention wherein the electro-optically FBG is fabricated by a thermal-poling process.
It is yet another object of the present invention wherein the gain bandwidth of the doped fiber is limited to a frequency range of the width of the wavelength spectrum associated with the doped fiber""s intracavity florescence and centered at the center frequency of the narrowband FBG.
It is yet another object of the present invention wherein the fiber laser can be actively mode-locked by biasing the reflection spectrum of the electro-optically tunable FBG at a particular biasing frequency and electro-optically modulating the spectrum at a particular cavity round trip frequency.
It is yet another object of the present invention wherein the first FBG is an electro-optically tunable FBG and the second FBG is an electro-optically tunable FBG, so that any frequency within the wavelength range of the gain spectrum of the doped fiber can be selected to be mode-locked.
It is yet another objection of the present invention wherein the selection of any frequency within the wavelength range of the gain spectrum of the doped fiber can be accomplished by stacking multiple fixed narrowband gratings.
In a first broad aspect, the present inventino provides a fiber segment comprising: a first fiber Bragg grating (FBG) located at a proximal end of the fiber segment; and a second fiber Bragg grating (FBG) located at a distal end of the fiber segment; wherein, when light enters said fiber segment at a first wavelength at said proximal end, a resonant cavity is created between said first FBG and said second FBG, whereby an output is mode-locked at a second wavelength and is produced at said distal end.
In a second broad aspect, the present invention provides a device comprising a plurality of fiber segments, wherein each fiber segment comprises: a doped gain medium; a first fiber Bragg grating (FBG) located at a proximal end of the fiber segment; and a second fiber Bragg grating (FBG) located at a distal end of the fiber segment; wherein, when light enters the fiber segment at a first wavelength at the proximal end, a resonant cavity is created between a first FBG and at least a second FBG, whereby an output is mode-locked at a second wavelength and is produced at the distal end.
In a third broad aspect, the present invention provides a device comprising: a light source; and a fiber segment; wherein the light source is aligned with the fiber segment, and wherein the light source is mode-locked within the fiber segment.
In a fourth broad aspect, the present invention provides a laser comprising: a light source; and a fiber segment; wherein the light source is aligned with the fiber segment, and wherein the laser is mode-locked.
Other objects and features of the present invention will be apparent from the following detailed description of the preferred embodiment.